1. Field of the Invention
The present invention relates to a power amplifier, and more particularly to a power amplifier having an amplifier circuit including a plurality of heterojunction bipolar transistors connected in parallel to one another.
2. Description of the Background Art
Along with the recent advancements in communications devices, there is a significantly growing demand for high-frequency, high-power devices. Particularly, mobile telephones require high-power devices that operate with high efficiencies in order to realize both a high-power operation and a long talk time using a battery. Thus, there is needed a high-power device capable of efficiently converting an input DC power to a high-frequency power while suppressing the internal power loss as much as possible.
Field effect transistors using a GaAs semiconductor are widely employed in high-power devices meeting such demands. An approach that has been attracting public attention in recent years is to use heterojunction bipolar transistors (hereinafter referred to also as “HBTs”) in high-power devices. An HBT is a bipolar transistor using a heterojunction of a compound semiconductor, and has desirable high-frequency characteristics and a high current-driving capability.
A high-frequency, high-power device includes a plurality of transistors connected in parallel to one another on a semiconductor substrate in order to output a high-frequency high power. FIG. 7 is a circuit diagram showing a configuration of a power amplifier including a plurality of HBTs as disclosed in U.S. Pat. No. 5,608,353. Referring to FIG. 7, a power amplifier 9 includes three amplifier transistors Q1 to Q3, each being a heterojunction bipolar transistor (HBT). The base terminals of the amplifier transistors Q1 to Q3 are connected to an input terminal PIN via capacitors CB1 to CB3, respectively.
A predetermined bias voltage is supplied to the base terminals of the amplifier transistors Q1 to Q3. The bias voltage is produced by resistively dividing the voltage applied to a bias power supply terminal VB with a resistor RB0 and a bias-producing transistor QB0. The bias-producing transistor QB0 is a heterojunction bipolar transistor (HBT), as are the amplifier transistors Q1 to Q3. The collector terminal and the base terminal of the bias-producing transistor QB0 are connected to each other. Thus, the bias-producing transistor QB0 functions as a diode.
In a power amplifier, a portion of a DC power provided as the power supply or a portion of a high-frequency power outputted as a signal is converted to heat, thus resulting in a power loss. In a power amplifier including a plurality of HBTs, heat from each HBT interferes with heat from other HBTs, and heat is localized at a particular HBT, thereby causing a phenomenon in which the collector current increases in the HBT at which heat is localized (hereinafter referred to as the “collapse phenomenon”). With such a collapse phenomenon, the HBTs no longer operate uniformly, thus reducing the gain and the power efficiency thereof, and possibly deteriorating or breaking the device.
In the power amplifier 9, resistors RB1 to RB3 are provided, for preventing thermal runaway, between the bias circuit (including the resistor RB0 and the bias-producing transistor QB0) and the base terminals of the amplifier transistors Q1 to Q3, respectively. The bias-producing transistor QB0 is an HBT having the same temperature characteristics as those of the amplifier transistors Q1 to Q3. Therefore, even if the characteristics of the amplifier transistors Q1 to Q3 change, the change can be canceled out by the change in the characteristics of the bias circuit. Thus, with the power amplifier 9, all HBTs can operate uniformly, thereby preventing the occurrence of the collapse phenomenon.
However, the conventional power amplifier described above includes a resistor provided in series with the base terminal of each HBT. If the base current increases during a high-power operation, the base potential is decreased due to the resistor, thereby decreasing the output power.